The present invention concerns a process for welding together a cover and vessel of thermoplastic material. The invention relates also to a cover, for application of the process. The process is especially suitable for covers and vessels of substantially rectangular configuration and is especially advantageous when either or both of the objects that are to be welded together present protrusions that project up or down from the vessel or cover, i.e., that pass through the surface that is defined by the edges that are to be joined.
Hot plate welding, i.e., heating against a heated metallic plate, is a method frequently used for joining a cover and vessel of thermoplastic material. Basically, the edges to be joined are heated to a molten state by the plate and then joined together. An important field of application for this method is the fastening of a cover on a cell vessel for electric storage batteries. The invention is especially suited for use in such a situation.
Formerly in hot tool welding, welding plates were used that covered and heated the whole surface defined and bordered by the edges against which the joining was to occur. This led to relatively large plates, for which considerable amounts of heat were used in heating them. Another drawback is that when the heating of the vessel edges is completed, the welding tool has to be removed by a relatively long time consuming operation motion. Another disadvantage of known methods is that there have to be concavities in the tool, corresponding to any parts or protrusions of the vessel and cover that extend upward or downward. This has the result that a specific welding tool can be used only for joining a cover and vessel with a specific, corresponding configuration, so that a shift to different types of vessel and cover is unnecessarily demanding in terms of cost and labor.
By means of the process of the present invention, there is a substantial shortening of the time for removal of the welding tool from vessel and cover after completion of the heating. In this way, there is better control of the temperature of the material surfaces at the moment of joining, whereby the quality of the obtained join can be improved. It is important for battery cells, for instance, to have a wholly tight weld. The process of the invention offers also considerably greater flexibility in the types of devices used for its execution.